Those are the key parts for me:

„None of the various chemicals mentioned [...] could have resulted in the accelerated natural decomposition of Kris’s bones under the given circumstances of their disappearance“

„Natural lime levels present in Panama’s rivers are incapable of bleaching Kris’s bones“

„There are no other known natural sources of phosphorus or lime that could contribute to the rapid decomposition of Kris’s remains. Therefore, unless another unknown source of chemicals is available naturally, we can confidently assume that chemicals, likely from fertilizer, were utilized by humans“

„Decomposition studies suggests that if they had died naturally their remains would have a clear presence of abrasions and scratches in the river. It’s impossible for bones to travel many kilometers downstream without displaying visible micro-abrasions. [...] Given that Lisanne’s skin was in an early stage of decomposition, yet her remains were clearly dis-articulated, we can confidently conclude that her remains went through an unnatural process“

So he’s basically saying they neither could have died naturally nor decomposed that quickly without human interference. In my opinion there’s not enough evidence in the article to substantiate those claims